Certain optical encoders for determining the relative position between two movable objects are conventional. It is possible to determine relative positions in linear movement directions as well as in rotary movement directions. In these systems, one object is usually connected with a measuring graduation while the other object is connected with a scanning unit. In the case of a linear encoder, a linear scale with a linear measuring graduation is used, whereas in the case of a rotary encoder, a code disk with a circular measuring graduation is used. The scanning unit used for either linear movement or rotary movement has one or more illumination sources and one or more optoelectronic detector elements. As detector elements, photodiodes are usually used.
In the last several years, linear and rotary encoders have become more and more popular having a plurality of interdigitized photodiodes as detector elements. Such a detector arrangement is also referred to a phased array or a structured photodetector. Following, the term detector array will be used.
It is conventional that these detector arrangements may have photodiodes arranged in an array on a semiconductor chip. The arrangement of the photodiodes has to be tailored for each encoder configuration in a unique manner. This means that the required geometrical arrangement of the photodiodes, such as their length, width and spacing, depends on the chosen scanning configuration, especially on the graduation period or graduation pitch of the scanned measuring graduation. For a certain measurement resolution—which is defined by the given graduation pitch of the measuring graduation—there exists a well-defined arrangement of photodiodes. Accordingly, if there is a need to change the resolution of the encoder by changing to a different scanned graduation pitch, there will be also a need to modify the design of the photodiode array in order to achieve the desired scanning configuration or resolution. An enormous amount of design work may be necessary to modify the layout of the photodiode array in this case.
To solve the above-mentioned problem, European Published Patent Application No. 0 710 819 describes the use of a detector array with a plurality of photodiodes for several different measuring graduations having different graduation periods. For that purpose, only a certain number of all available photodiodes has to be activated in dependence of the scanning graduation. An adaptation procedure is necessary to determine in each case which of the photodiodes have to be activated for a certain scanning graduation. One drawback of this system is that it may require a complex ASIC to control the adaption procedure. Another disadvantage is that the system's activation phase may require special tooling discs should light be allowed to shine on multiple incremental data signal groups. Furthermore, a lot of space for memories and associated circuitry on the carrier substrate may be necessary which is contrary to a possible miniaturization of the system.
Therefore, U.S. Pat. No. 6,727,493, which is expressly incorporated herein in its entirety by reference thereto, describes a detector array with an easily changeable resolution. For that purpose, a resolution selection unit is connected to the photodiode array and is used to control the resolution of the array, wherein all photodiodes associated with the photodiode array are active irrespective of the actual resolution selected by the resolution selection unit. In the case of a rotary device, this arrangement allows switching between different resolutions or different code disc pitches at a defined rotary scale radius.
U.S. Pat. No. 6,727,493 does not offer a solution, however, if an encoder with a rotary scale on the code disc having a second radius is necessary for another application in which the first and second radii are different. In this case, a different photodiode array has to be designed for each rotary scale radius again. This requires a further amount of design work necessary to modify the layout of the photodiode array so that it can be used to scan a rotary scale on a code disc with at least a second rotary scale radius.
Similar problems arise if index marks optimized for a certain scale radius have to be scanned by a suitable photodiode detector array. Index marks on the code disc are usually used to generate so-called reference signals indicating a certain defined absolute position along the measuring graduation. In this case it also requires new design work if a photodiode detector array has to be optimized for different code disc radii.